NITROGEN AND WATER FLOWS UNDER PASTURE-WHEAT AND LUPIN-WHEAT ROTATIONS IN DEEP SANDS IN WESTERN AUSTRALIA. 2. DRAINAGE AND NITRATE LEACHING

Quantiflcation of nitrate (NO i ) leaching is fundamental to understanding the e-ciency with which plants use soil-derived nitrogen (N). A deep sand located in the northern wheatbelt of Western Australia was maintained under a lupin (Lupinus angustifolius){wheat (Triticum aestivum) and a subterranean clover (Trifolium subterraneum) based annual pasture{wheat rotation from 1994 to 1996. Fluxes of water and NO i through, and beyond, the root-zone were examined. Drainage was calculated on a daily basis from measurements of rainfall, evapotranspiration, and the change in soil water content to a depth of 1¢5 m. Evapotranspiration was estimated from Bowen ratio measurements, and soil water content was determined by time domain re∞ectrometry. Soil was sampled in layers to 1¢5 m at the onset of winter rains and analysed for NO i . Ceramic suction cups were installed at 0¢25, 0¢4, 0¢6, 0¢8, 1¢0, 1¢2, and 1¢4 m to sample soil solution from June to mid August. The NO i leached from each layer was computed by multiplying the daily drainage through each layer by the estimated concentration of NO i within the layer. The estimated concentration of NO i in al ayer was calculated by taking into account NO i either entering that layer through mineralisation and leaching or leaving the layer through plant uptake. Mineral N was added to the surface 0¢2 m in accordance with measured rates of net N mineralisation, and daily N uptake was calculated from the measured above-ground plant N derived from soil N. Root sampling was undertaken to determine root length density under pastures, lupin, and wheat. Cumulative drainage below 1¢5 m was similar under wheat and lupin, and accounted for 214 mm from 11 May to 15 August 1995 and 114 mm from 2 July to 15 September 1996. The cumulative evapotranspiration (Ea) over these periods was 169 mm from a wheat crop in 1995, and 178 mm from a lupin crop in 1996. The amount of NO i in soil at the start of the growing season was afiected by previous crop, with a lower range following wheat (31{68 kg N/ha) than following legumes (40{106 kg N/ha). These large quantities of NO i in the soil at the break of the season contributed substantially to NO i leaching. Leaching of NO i below 1¢5 m in wheat crops accounted for 40{59 kg N/ha where these followed either lupin or pasture. In contrast, less NO i was found to leach below 1¢ 5m in pastures (17{28 kg N/ha). Greater N uptake by capeweed (Arctotheca calendula L.) than by either wheat or lupin was the main reason for the lower amount of NO i leached in pastures.